LibGUI+WindowServer: Provide default placement to windows

This prevents windows from being opened directly on top of eachother,
and provides default behavior for when window position is not specified.

The new behavior is as follows:
- Windows that have been created without a set position are assigned one
  by WindowServer.
- The assigned position is either offset from the last window that is
  still in an assigned position, or a default position if no such window
  is available.

Services/WindowServer/Window.cpp

@@ -273,6 +273,7 @@ void Window::set_maximized(bool maximized)
     }
     m_frame.did_set_maximized({}, maximized);
     Core::EventLoop::current().post_event(*this, make<ResizeEvent>(old_rect, m_rect));
+    set_default_positioned(false);
 }
 
 void Window::set_resizable(bool resizable)
@@ -398,7 +399,7 @@ Window* Window::is_blocked_by_modal_window()
         if (window && !window->is_destroyed()) {
             if (window->is_modal())
                 return window;
-            
+
             if (auto* blocking_modal_window = window->is_blocked_by_modal_window())
                 return blocking_modal_window;
         }
@@ -601,7 +602,7 @@ bool Window::is_accessory() const
         return false;
     if (parent_window() != nullptr)
         return true;
-    
+
     // If accessory window was unparented, convert to a regular window
     const_cast<Window*>(this)->set_accessory(false);
     return false;